Dual digestion process for sewage



. Iuly 12, 1938. I M, B, TARK 2,123,387

DUAL DIGESTION PROQESS FOR SEWAQE Filed March 2l, 1935 I Patented Julyv 1.2, 1938 UNITED s'rll'rus PATENT AoFFIc-u y 2,123,381 DUAL pIGEs'rroN Paoosss non sEwAGn Mai-cus mark, Philadelphia, ra, assigner to Link-Belt Company, Chicago, Ill., a corporation Application March 21, 1935,"sem1 No. 12,207

1s claims. .'(cl.v 21o-2) My invention relates toimprovements in ape.

paratus for and process of treating sewage material and the like,` and has for one object to improve the-digestion ofchemically;L precipitated 5 sludge which is much morefdimcultio digest than is the ordinary sludge from a primary settling T out. The chemically precipitated sludge from theV secondary settling tank will be discharged vinto a secondary chemical sludge digestion tank,

while the eiliuent from the secondary settling tank will be decanted vin the usual manner.

'I'he sludge resulting from chemical precipitation does not vdigestas readily as does the sludge from the primary settling tank because of the presence of the chemicals therein, and so it is necessary to take special precaution and provide special means to control and promote the diges- .tion oi' the chemical' sludge. vIn order to accom- 'plish this, I propose to make use of the digested sludge from the primaryA digestion tank.

The percentage oi' solids in theI sludge obtained from -the primary settlingtank is much'higher than the percentage of solids 'in the sludge obtalnedfrom the secondary lsettling tank in mydigested sludge which contains a maximum of 'liv`e and active bacteria so as to'seed the sludge in the secondary digestion tank and to maintain la properhealthy condition inthe bacteria carrylng on the digestive processes therein. Experi- -ence has taught that in order to Promote satisfactory dlgestiomlt isonecessary tovhave present in the digestion chamber ralong with the sludge undergoing' digestion, a considerable amount of completely digestedsludge which gives whatvsevv-l age engineers call the buier effect. The excess 55. of. digested sludge from the primary digestion digestion process.

tank supplied to the secondary digestion tank furnishes this exeessof digested sludge and gives this essential buier effect.

It has heretofore been suggestedthat the digestion of the sludge from' the primary and the secondary or chemical settling tanks can take place in a single digestion chamber, 4the two sludges being mixed together to undergo the My method is an improvement'over s uch a solution of the problem because I have found that when an attempt ismade to digest together' untreated sludge and chemicallyprecipitated sludge, the chemicalsy in the chemically precipitated sludge which tend to make digestion of that sludge diilicult also have a noticeable deleterious effect on thedige'stlon of the untreated sludge with which they are rixed. Y By carrying on the digestion of the untreated sludge from the primary settling tank .to completion by itself, I' am enabled to obtain normal satisfactory digestion, and once that digestion has been accomplished, the digested sludge can be supplied to the secondary-chemical sludge digestiontank without any deleterious effect on the product or operation of the primary tank and results in a greatly improved eect both in the operation andthe product of the secondary diges- .tion tank.

" .When, as is frequently thelcase, digestion is promoted in both tanks by heating., it may be desirable to have an additional settling tank so that sludge particles maintained in suspension in the super-natant liquor by eddy currents, convection and the like, may be settled out, and this settling tank also serves as an equalizing chamber so thatlwhen sludge is added to or withdrawn from the digestion tanks, this may be compensated for by a ilow of super-natant. liquor from or to the settling tank to prevent change in the level of the liquor in the digestion tanks, which might otherwise interfere with the-proper digestion process. s' l Where I have referredto a ilocculator, it will of course be understood that thel dosage or chemicals'supplied in the occulator tank to the efiiuent from the primary settling tank -might equally well be added directly in the settling tank orby any other suitable means and their leiIect is of course to coagulate the sewage lso that lthe primary settling tank maybe said to serve to pre-clarify. the sewage.

My invention is illustrated in the 'accompanying drawing, wherein is shown a' diagrammatic lay-out of aplant'adapted tocarry out my'invention.- l f .Y

soV

I is the outfall sewer; 2 a primary settling tank; 3 a sludge pipe leading from the primary settling tank to the primary digestion tank 4. 5 is a sludge pump adapted to withdraw the sludge from the primary settling tank 2, and supply it to the primary digestion tank 4.

6 is an'eiiiuent pipeleading from the primary settling tank 2, to the mixing and locculating and dosing tank 1. 8 is the chemical supplying and measuring means. 9`is a pipe leading from the ilocculating tank 'l to a secondary settling tank I; and II is an eiliuent passage adapted to discharge the eilluent from the secondary settling tank IIL I2 is a sludge pipe leading from the secondary settling tank to the secondary digestion tank I3. I4 is a sludge pump associated with said pipe. I is a sludge pipe leading from the bottom of the primary digestion tank 4, to the secondary digestion tank I3. IB is a pump associated with the pipe I5. I 1 and I8 are supernatant liquor pipes leading respectively from the primary digestion tank 4, and the secondary digestion tank I3 to the settling tank and equalizing tank I9. Suitable valves are placed in the various pipes to control ow through vth'em in the usual manner. y

Preferably the ilow to and through the pri# mary and secondary settling tanks and the ocf culation tank will b by gravity.l Normally the sludge will be withdrawn from each of the two settling tanks andfed to the respective digestion tanks by the pumps, though under some circumstances, the terrain may make it possible to withdraw the sludge by gravity. The same is true with respect to the movement of sludge from the primary to the secondary digestim` tanks. The iiow or super-natant liquor between the two digestion tanks and the settling and equalizing tank I9 will normally be by gravity.

Sludge from the tanks 4, I3, and I9 will be discharged through the sludge discharge system 20, preferably by gravity though pumps not here shown may be provided if necessary, controlled by the valves as shown, for application to sludge drying beds or other suitable points of disposal.

In operation the flow through the primary settling tank, the occula-ting tank and the secondary settling tank will normally be continuous with continuous passage of the eiluent. from the primary settling tank. Periodically the 'y sludge will be withdrawn from the bottom of the two settling tanks and discharged respectively from the primary and the secondary digestion tank. Digestion in the primary digestion tank will take place in the usual manner and digested sludge will be deposited on the bottom of the tank. This digested sludge will be periodically withdrawn from the bottom of the primary digestion tank and supplied to the secondary digestion tank, care beingstaken that only digested sludge is so supplied: This digested sludge, carrying with it an excess supply of live, active bacteria and the like, will be added to the secondary digestion tank and there mixed with a relatively thin chemicallylprecipitated sludge, seeding the same and yfurnishing the excess of digested sludge which gives the essential buier effect.

When sludge is fed to either of the two digestion tanks, super-natant liquor will be displaced and that super-natant liquor will iiow through .the pipes I'I and I3 as the case may'beand be trapped in. the settling and. equalizing tank i9. When sludge is withdrawn from eitherv of the two digestion tanks, super-natant liquor from the tank I9 will ow into the digestion tank to maintain the level substantially constant.

When as will frequently be the case, the digestion tanks 4 and I3 are heated to promote digestion, there will be a certain amount of movement by convection of the super-natant liquor and sludge particles contained therein so that some of the sludge will not settle out to the bottom of the tank even though it has been completely digested. When it is desired to draw off super-natant liquor fromv either of the two digestion tanks, that liquor will pass to the settling tank I9 where it will be cooled, the digested sludgesettling to the bottom of the tank whence it may be subsequently Withdrawn to the sludge beds, the super-natant liquor being there clarified, may be discharged without objection.

The sludge deposited on the bottom of the primary tank 4, which is in excess of that which must be added to the secondary tank I3, may also be drawn off when desired and the same is true of the sludge deposited at the bottom of the secondary digestion tank, the sludge from all three tanks being handled and treated in the usual manner and being available to be placed on the drying beds or otherwise disposed of.

2l, 22, and 23 are valved super-natant liquor pipes leading respectively from the upper portions of the digestion tanks I3 and 4 and the settling tank I9 so that means are provided in each tank to withdraw both sludge and supernatant liquor immediately from the system if that should prove desirable.

I claim:

1.4 Apparatus for use in the treatment of sewage which comprises a pre-clarifying tank having an inlet for raw sewage, anv outlet for pre-v claried sewage and an outlet for settling sludge, a digester connected to said sludge outlet for the digestion of sludge, a settling tank connected to the said outlet for pre-clarified sewage, means for introducing a chemical coagulant into said pre-clarified sewage, an outlet connected with said settling tank for leading oif the clear sewage eilluent, a second digester and means for conducting sludge precipitated in the settling tank thereto for digestion and means for conducting digested sludge from the iirst digester to the second.

v2. A 4continuous process of treating sewage which comprises establishing a continuous ow of raw sewage, subjecting said lsewage to preclarication to obtain a sludge and pre-clarified sewage, withdrawing the thus pre-clarified sewage and treating it after said withdrawal with coagulants to cause precipitation of sludge from the thus treated pre-clarified sewage, drawing off the effluent, exposing the pre-clarification sludge to digestion treatm'ent, introducing the digested sludge together with chemically precipitated sludge into a second digestion tank and exposing the chemically precipitated sludge to a process of digestion.

3. The-process of treating sewage which consists in passing it through a primary settling zone, withdrawing the solids there settled out as sludge and causing their digestion in a primary digestion zone, decanting the eilluent from the primaryfsettling zone, mixingit with chemicals adapted to promote ilocculation and precipita'- tion of the remaining solids and colloids, passing the decanted liquid through a secondary settling zone, withdrawing the solids there settled out as chemically precipitated sludge and causing their digestion in a secondary digestion zone, decanting Vzo the eilluent from the secondary settling zone, withdrawing digested sludge from the primary digestion zone and adding it to the sludge in the secondary digestion zone.

4. The process of treating sewage which consists in passing it through a primary settling zone, withdrawing the solids there settled out as sludge and causing their digestion in a primary digestion zone, decanting the eiiiuent from the primary settling zone, mixing it with chemicals adapted to promote flocculation and precipitation of the re. maining solids and collo/ids,l passing the decanted liquid through a secondary settling zone, withdrawing the solids there settled out as chemically precipitated sludge and causing their digestion in a secondary' gestion zone, decanting the eilluent from vthe secondary settling zone, withdrawing digested sludge from the primary digestion zone and adding it to the sludge in the secondary digestion zone, the mass of digested sludge added to the secondary digestion zone from the primary zone being relatively large with respect to the chemically precipitated sludge.

p 5. 'I'he process of treating sewage which consists in passing it through a primary settling zone, withdrawing the solids there settled out as sludge and causing their digestion in a primary digestion zone, decanting the eilluent from the primary settling zone, mixing it with chemicals adapted to promote ilocculation and precipitation of the remaining solids and colloids, passing the decanted liquid through a secondary settling zone, withdrawing the solids there settled out as chenilcally precipitated sludge and causing their digestion in a secondary digestion zone, decanting the eiiiuent from the secondary settling zone, withdrawing digested sludge from the primary digestion zone and adding it to the sludge .in the `secondary digestion zone, withdrawing supernatant liquor from the digestion zone to a settling zone and there settling out the sludge.

6.l A sewage disposal plant comprising primary and secondary settling tanks, means for supplying sewage liquid to the primary and decanting it therefrom to the secondary settling tank, means for supplying a ilocculating chemical to the decanted liquid, a primary digestion tank and means for supplying it with settled sludge from the'primary settling tank, a secondary/.digestiontank -and means for supplying it with settled sludge from the secondary settling tank, means i'or supplying digested sludge from the primary digestion tank to the secondary digestion tank, a settling chamber and means connecting the settling chamber with each of the digestion tanks.

'1. A sewage disposal plant comprising a primary and a secondary` digestion tank, a primary' and a secondary settling tank, a chemical dosage tank and a settling tank for supernatant liquor,

a conduit adapted to bring raw sewage to the primary settling tank, -a conduit adapted to convey decantedl sewage therefrom to the dosage tank and a conduit adapted to convey chemically treated sewage from the dosage tank to the secondary settling tank, separate conduits and pumps adapted to convey settled sludge from the primary settling to the primary digestion tank and from the secondary settling to the secondary digestion tank, av conduit and pump adapted to convey digested sludge from the primary to the secondary digestion tank, and means for connecting the settling tank for supernatant liquor with the primary digestion tank.v

8. A sewage disposal plant comprising a primary and a secondary digestion tank, a primary and a secondary settling tank, a chemical dosage tank and a sludge settling tank, a conduit adapted to bring raw sewage to the primary settling tank, a conduit adapted to convey decanted sewage therefrom to the dosage tank and a conduit adapted to convey chemically treated sewage from the dosage tank to the secondary set- I tling tank, separate conduits adapted to convey settled sludge from the primary settling to the primary digestion tank and from the secondary settling to the secondary digestion tank, a conduit adapted to convey digested sludge from the primary to the secondary digestion tank and conduits adapted to connect the two digestion tanks and the sludge settling tank.

9. In a sewage treatment apparatus, plural settling tanks, means for delivering sewage to. both tanks, plural digesters each digester being connected to a settling tank, means for delivering settled sludge from one tank to one of the digesters and means for concurrently delivering settled sludge from another tank to another digester, and a supernatant liquor settling chamber connected with said digesters to receive the supernatant liquor from any one of said digesters.

l0. In a sewage treatment apparatus, plural settling tanks, means for delivering raw sewage to one of said tanks, means'i'or delivering chemi- `cally treated sewage to another of said tanks,

means for segregating and separately digesting the sludge settling from the respective tanks said means including a plurality of digesters and connection from each of said digesters to one of said settling tanks, a common settling chamber for supernatant liquor resulting from said digestion, and means connecting said chamber with said digesters.

11. In a sewage treatment apparatus, the ap-I paratus described inclaim 10, wherein means are provided for bypassing the contents of any` digester around said settling chamber to a. point of MARCUS B. T ARK. 

